This invention relates generally to a system for sorting particulate material. More particularly, this invention pertains to a method and an apparatus for separating a mixture of particles according to specific gravity.
One specific purpose of the invention is to remove lightweight deleterious impurities such as particles of coal from sand destined for use in concrete and mortar and bring the level of such deleterious material down to an acceptable level. There is an ongoing need for reasonably clean sand in order to provide essential ingredients for concrete, mortar and asphalt products. Typically medium sized sand is used for concrete, a finer mix for mortar and a combination of sizes for asphalt. When sand is quarried from deposits in a raw form, it usually contains unwanted components including rocks of various sizes, vegetable matter, clay, silt, slime and various lightweight materials inter alia coal (anthracite and lignite) and shale. All these materials generally need to be removed. It is particularly important, for several reasons, to remove lightweight particles such as coal when the sand is destined for use in concrete and mortar. The primary reason is that coal is a significantly softer material than sand. If not removed, the hardened concrete or mortar will have the coal particles therein, and these particles quickly erode and degrade resulting in voids within or on the surface of the finished concrete or mortar. These voids fill with water, and freezing and melting cycles can quickly degrade the concrete or mortar. The coal is also more porous than sand and tends to absorb water even before degrading, thus promoting destructive freezing and melting cycles and early damage to the concrete or mortar. Additionally, the coal also tends to smear when the concrete is worked and tooled, causing unsightly black streaking of the concrete or masonry surface. These problems make it important to remove the coal during the cleaning cycle.
State of the art sand processing begins with the quarried, raw sand being run through one or more conventional, superposed screens having decreasing mesh openings in which rocks of various sizes, vegetable matter, and any other fairly large impurities are separated from the raw sand. During this screening, a working fluid is often added to the raw sand to form a slurry. With sand processing, the working fluid will be water. The slurry produced during this preliminary step is next directed to a classification tank in which the raw sand is allowed to settle to the bottom of the classification tank and the lighter clay, silt, and slime is washed away with the water.
Generally, classification tanks are rectangular and include an inlet end, an outlet end and a plurality of settling stations (typically 7-11) positioned between the inlet and outlet ends along a bottom surface. There are usually no physical dividers between these stations, and the first two to three stations at the inlet end will be spaced more closely to each other than will the last stations at the outlet end of the tank. The raw sand, after being preliminarily screened and washed as described above, will be directed into the classification tank in a slurry form, entering the tank at a predetermined rate. As the slurry moves along the classification tank, the raw sand carried by the slurry is transported along the classification tank with the heaviest sand particles dropping to the bottom at the first two or three settling stations and the medium and smaller sand settling at subsequent settling stations. Clay, slime and silt tend to remain in suspension and are carried away by the water flow as it exits the outlet end of the classification tank.
Each settling station is usually operatively connected to three drainpipes, and each of these pipes discharge to one of three distinct and adjacent flumes positioned beneath the tank. The drainpipes at these settling stations are normally kept closed by remotely operated valves and the valves are opened at predetermined times to discharge the accumulated sand particles of that station. A control system determines which of the three valves at each settling station to open, and as a result the collected sand at that station may be directed to a particular flume. For example, the coarsest particles may be directed to the first flume, the medium particles directed to the second flume and the fine particles directed to the third flume.
Until the present invention, it was quite challenging to remove lightweight impurities from the slurry, and previous attempts to do so proved less than satisfactory. In one attempt, a jig that was used to separate lightweight particulate matter from larger, stone sized particles was modified for use with smaller, sand sized particles. A typical jig consists of a nearly horizontal bed over which a very shallow flow of slurry is directed. As the slurry traverses the bed, the bed is vibrated, and this vibration causes the lightweight material to rise to the top of the slurry. When the slurry reaches the end of the bed, the lightweight material is skimmed off from the upper portion of the slurry by a knife the desirable particles are allowed to pass beneath. The knife adaptation of this device for sand sized particles has met with only limited success. For example, the capacity of such a modified jig is inherently limited because if the slurry is too deep, the lightweight particles will not be able to be brought up to the surface by the vibrating bed. Conversely, it is not very efficient to have a relatively shallow slurry depth because only small amounts of material may be processed. Moreover, such a modified jig is a complicated and expensive piece of equipment that is costly to operate. Additionally, and more importantly, such a jig is unable to effectively fractionate the raw sand feed material and process only that portion in which lightweight particles are concentrated.
Another apparatus known to the art to separate lightweight particles from denser particles used a density separator. A density separator is a device that also operates on the principle of floatation. A typical density separator is a large, square, open-top tank having a bottom with a single discharge valve. The open top of the tank includes weirs over which a working fluid and suspended material is permitted to flow. A slurry is introduced into the tank and fills it to overflowing. By adjusting the flow rate, it is possible to suspend and remove lightweight material as the upwelling fluid flow passes over the weirs. This overflow is typically passed over a static screen to remove larger lightweight materials, and the finer material and working fluid which pass through the screen may be directed to other areas for further processing. The coarser material settles to the bottom of the tank and is periodically directed to other areas for further processing. As with the aforementioned modification of an existing jig, use of a density separator to separate and remove lightweight materials from smaller, sand sized particles has met with limited success. Capacity is limited. For example, if the slurry is inputted in to the tank at too great a rate, both undesirable and desirable material may flow over the weirs and onto the screen. All of this discharge tends to clog the screen and reduce its efficiency. Moreover, not all of the lightweight material is effectively separated from the sand particles, and lightweight material can become trapped along with the coarser particles which settle at the bottom of the tank. If the slurry is inputted too slowly, it will not be possible to induce all of the lightweights to flow over the weirs and excessive lightweight material settles at the bottom of the tank along with the coarser sand particles. This is not only limiting, it is also inefficient. Another inefficiency occurs at the single screen through which all of the overflow material is directed. And, as with the modified, aforementioned jig, this device is unable to fractionate the raw feed material and process only that portion in which lightweight particles are concentrated. That is, there is no effective way to control the settling patterns of the particulate material as it settles on the bottom of the tank, and there is no provision to process portions of the settled particulate matter differently.
Yet another attempt, a field retrofit, used a classification tank in conjunction with a single static wire cloth screen through which all of the material drained from a predetermined settling station was directed. This had several drawbacks. It was not very efficient because the lightweight material prevented the desirable sand particles from passing thereby and tended to attract particles of sand to form clumps which further clogged the screen. This reduced the processing speed and efficiency of the screen and required repeated and frequent down time to unclog. Moreover, not all of the sand needed to be screened, which also contributed to the reduced processing efficiency of the screen.
In an attempt to reduce the clumping and clogging, the aforementioned static screen was vibrated by a motor that was operatively connected thereto, and which rotated at a rate of around 1,000 revolutions-per-minute (RPM). This improved the processing efficiency of the screen somewhat by reducing clumping and clogging, and with it some of the lightweight contaminants were separated from the sand. However, it was observed that the resultant end product still contained significant lightweight contaminants. More importantly, these lightweight contaminants were now mixed with the end product and were now more difficult to remove from the end product and required additional, costly and time-consuming processing.
Both of these attempts also were unable to fractionate the raw material and these structures processed only a single settling station where lightweight materials were seen to be more concentrated.
The invention includes an apparatus and method for sorting a mixture of particles according to weight and size. The apparatus includes a classification tank having an inlet, an outlet and a plurality of settling stations located therebetween. A slurry of working fluid and particles is introduced into the inlet and directed towards the outlet. As the slurry traverses along the classification tank, particles come out of suspension and settle at the settling stations according to weight and to a lesser degree, density. Thus, the heavier particles tend to settle out before the medium and fine particles, as in a continuum. The composition as well as the quantity of particles that have settled at the settling stations is periodically inspected. If particles deposited at a particular settling station are approximately the same size, and if they have accumulated in sufficient quantity, they are allowed to discharge downwardly therefrom into a flume which directs them to preselected collection areas.
While the invention may be utilized with a number of different lightweight contaminants to remove the contaminants, coal, specifically lignite and anthracite contamination is of the greatest concern. Coal has a specific gravity which is about half that of sand used in concrete and mortar. In a typical classification tank, coal particles are often heavy enough to settle to the bottom and mix with the various sand particles rather than remain in suspension. It has been observed that a coal particle which settles at any given settling station is usually about twice or more the size of the adjacent sand particles. It has also been observed that the coal particles of varying sizes carried by the slurry will settle in a similar fashion along a number of different settling stations. It has also been observed that the stations at which settling of the coal particles occurs are not wholly predictable, but with a given source of raw sand, the settling patterns are usually reasonably constant, but do need to be identified by testing. A part of the present invention includes identifying the settling stations of the classification tank at which significant lightweight particle settling occurs and then diverting the particles which settle at those settling stations to a second stage where the larger (and less dense) lightweight particles, such as coal, are separated from the sand by screening.
Thus, if the particles deposited at a particular settling station vary substantially in size, which would indicate the presence of unwanted lightweight material such as coal, and if such lightweight material has accumulated in sufficient quantity, the collected deposited material is allowed to discharge therefrom into a different flume or flumes which directs it to a screening station or stations prior to directing it to preselected collection areas.
Part of the present invention also includes equipping the classification tank with one or more rising current classifier cells which are positioned in the classification tank adjacent to the input end. Each such cell contains a grate through which an upward current of working fluid such as water may be directed. The current may be varied so that it has a flow rate which is sufficiently strong to urge the lightweights upward and away from the grate, but which is not strong enough as to prevent the heavy coarse sand from dropping downward through the grate to the bottom of the first three stations. The cells and their use are an important structure and step which immediately separates the lightweight particles from the coarse sand as the slurry enters the input end of the classification tankxe2x80x94and most of the sand is coarse sand. As a result, the amount of remaining sand that may require lightweight particle removal, is greatly diminished.
In removing the sand accumulated at the bottom of the settling stations, it has been found that it is desirable to treat, as a group, all the settling stations that contain medium size sand and to divert all such sand at these stations to a first flume, which will lead to a first screen where the lightweight particles associated with this first group of stations will be removed. The remaining stations where fine sand has settled will also be treated as a group, and the accumulated fine sand and lightweight particles at these stations will be discharged to a second flume, which will lead to a second and finer screen where the lightweight particles associated with those stations will be removed. The coarsest particles, which will settle out at the first three settling stations is kept relatively clear of lightweight particles by use of the classifier cells near the input end of the classification tank so as to increase upward lift and prevent even larger lightweight particles from settling there. As a result the coarse sand at settling stations 1-3 has little deleterious material therein and can be drained to a third flume and accumulated without need for further screening. The smallest sand particles, which settle near the outlet end of the classification tank will be discharged to a fourth flume and accumulated without the need for further processing. Thus configured, the apparatus and method is able process large amounts of raw material in an efficient manner while reducing the need for elaborate designs and avoiding costly equipment and operational expenses.
It is an object of the present invention to provide a system that separates particles of material by weight and size.
Another object of the invention to provide a system for separating deleterious, low density particles from primary, higher density particles.
Yet another object of the present invention is to streamline the separation process by controlling the deposition gradient in a classification tank and by determining if portions of the deposition gradient require additional processing.
A feature of the present invention is the use of at least one screen downstream of the classification tank to separate particles by size.
Another feature of the present invention is the use of sprayers to facilitate particle separation at the at least one screen.
Another feature of the invention is the predetermined orientation and vibrational characteristics of the downstream screen.
Yet another feature of the present invention is the use of an auxiliary flow of fluid to prevent the accumulation of deleterious lightweight particles at predetermined locations of the classification tank.
An advantage of the present invention is that deleterious lightweight material may be effectively and efficiently separated from a primary material.
These and other objects, features and advantages of the present invention will become apparent from the following detailed description thereof taken in conjunction with the accompanying drawing, wherein like reference numerals designate like elements throughout the several views.